Induction heat treatment method and article treated thereby

ABSTRACT

A method of induction heat-treating certain surfaces of an article comprising a hub and a plurality of trunnions or shafts extending therefrom, wherein the surfaces to be heat-treated comprise the surfaces of the trunnions. The method employs an induction coil that provides a planar magnetic field that is adapted to the irregular shape of the article, such that the induction coil produces magnetic fields that are adapted to induce currents in, and thereby heat, the trunnion surfaces of the article. The method and induction coil are particularly adapted to provide an induction hardening heat treatment for trunnion surfaces of trunnions used in steel spiders of spider or tripod-type constant velocity joints.

FIELD OF THE INVENTION

The present invention relates to a method of induction heat treatment.More specifically, the invention comprises a method for inductionhardening certain metal components, particularly those having a shapethat prevents uniform induction coupling over the entire surface to behardened in a single step. Most particularly, the invention comprises amethod for induction hardening the outer surfaces of the trunnions of aspider of a steel tripod-type constant velocity joint.

BACKGROUND OF THE INVENTION

The tripod-type constant velocity joint is widely used in automotivevehicles. It is most frequently used to provide a plunging andangulating constant velocity joint for use with halfshafts, the namegiven to the two driveshafts or axle shafts that run from the transaxleto the wheels in front wheel drive vehicles. A plunging joint is onethat permits axial movement between the shafts. Because it is widelyused in automotive vehicles, the tripod joint is manufactured inrelatively high volumes. A tripod joint consists of a spider whichcomprises a hub, often with a splined bore in its center, and threeangularly-spaced shafts or trunnions extending from the hub, and threeroller bearings. Each roller bearing is fixed on an end of a trunnionand is adapted to slide in a corresponding groove that is cut into theinside surface of a tube-like housing. The power to the driven wheels ofa vehicle is transmitted through the trunnions and the point of contactbetween the three bearings and three grooves. Given the magnitude andcyclic nature of the loading of the trunnions, they have carefullydefined requirements for strength, toughness and fatigue resistance.

While variations exist in the designs for and materials used in spidersof different manufacturers, a spider is typically formed by forging froma pearlitic/ferritic steel blank, and is then subjected to various metalforming, finishing and heat treatment steps to produce a finishedarticle. In one example, the spider is specified as an AISI 1050warm-forged steel material. The required properties comprise a surfacehardness on the trunnions in the range of R_(C)58-63, with a hardenedcase depth of approximately 1.0-2.0 mm effective at R_(C)50, and a corehardness of R_(C)15-30. The microstructure is required to showmartensite, preferably tempered martensite, in the case with fine grainsof pearlite and ferrite in the core. The required hardness is necessaryto provide the strength in the load bearing areas of the outer surface,and the necessary toughness and fatigue resistance in the core.

Spiders of various configurations have previously been hardened bycarburizing to provide the necessary microstructural properties, such asthose described above. The use of carburizing for case hardening has anumber of well-known limitations. These include the fact that theprocess treats the entire surface of the component, the material andprocessing costs associated with the process, the processing timenecessary to heat the parts to temperature and produce the requiredcarburized case depth, as well as limitations related to processcontrol, batch processing, capital expense and facility requirements forlarge furnaces, environmental issues, and control of the finished partquality. Also, the carburizing process has the potential to formundesirable microstructures, which include carbides, grain boundaryoxidation, decarburization and retained austenite that can each affectthe functionality of the finished part. Therefore, it is desirable todevelop an improved method of heat treatment that addresses thelimitations mentioned above and that provides a method for surface orcase hardening a part such as a spider.

Induction heat treatment is known to be an effective method of casehardening pearlitic/ferritic steels and avoiding many of the limitationsmentioned above that are associated with carburizing. For example,induction hardening has been widely used for the case hardening ofvarious types of steel gears. However, induction hardening hassignificant limitations in cases where the surface requiring heattreatment is irregular, such as gears having relatively larger teethwhere the distance from the tip to the root of a tooth is such that theelectromagnetic coupling, and hence induction heating, variessignificantly from the tip to the root. While some solutions have beenproposed to facilitate the use of induction hardening with articleshaving irregular surfaces, such as the use of a plurality of differentcoils and different induction frequencies to treat different portions ofthe surface, or the use of coil designs that are adapted to the contourof the irregularities in order to provide more uniform coupling,induction hardening has not been used for various types of irregularlyshaped components, including spiders, perhaps because the use of thetechniques described above are not applicable to provide a single passinduction heat treatment of the critical surfaces of a spider due to itsirregular shape.

Therefore, it is desirable to develop an induction heat treatment methodthat can be utilized to provide induction hardening of irregularlyshaped components, particularly those that comprise a hub and aplurality of trunnions extending outwardly therefrom, such as thevarious spiders utilized for tripod-type constant velocity joints.

SUMMARY OF THE INVENTION

The present invention provides a method of induction heat treatment,comprising the steps of: (1) selecting an article, such as spider, forheat treatment comprising a hub having a hub surface and a plurality ofangularly spaced trunnions extending from a corresponding plurality oftrunnion shoulders formed in the hub surface, each trunnion shoulderhaving a trunnion shoulder surface, and each trunnion having a trunnionaxis and a trunnion surface; (2) selecting an induction coil, which isadapted to receive a trunnion for heat treatment and apply a magneticfield to the trunnion surface and trunnion shoulder surface; (3) placinga trunnion within the induction coil with its corresponding trunnionshoulder adjacent to the induction coil; (4) rotating the trunnionwithin the induction coil about the trunnion axis at a selected speed;(5) energizing the induction coil to apply the magnetic field andproduce induction currents within the trunnion shoulder surface andtrunnion surface of the article for a time sufficient to induce heatingthem to a heat treatment temperature (T_(H)) to at least a selected casedepth; (6) withdrawing the trunnion from the induction coil at aselected rate; (7) cooling the trunnion surface and trunnion shouldersurface of the article to a temperature (T_(C)) to the selected casedepth; and (8) repeating steps (3)-(7) for a selected number of thetrunnions.

More particularly, the present invention also provides a method ofinduction heat treatment of a spider having a barrel-shaped outersurface, a plurality of cylindrical trunnion shoulders formed in theouter surface of the hub and a corresponding plurality of angularlyspaced cylindrical trunnions extending from the shoulders, each trunnionshoulder having a trunnion shoulder surface, and each trunnion having atrunnion axis and a trunnion surface, comprising the steps of: (1)selecting an induction coil, which is adapted to receive a trunnion forheat treatment and apply a magnetic field to the trunnion surface andthe trunnion shoulder surface; (2) placing a trunnion within theinduction coil with its corresponding trunnion shoulder adjacent to theinduction coil; (3) rotating the trunnion within the induction coilabout the trunnion axis at a selected speed; (4) energizing theinduction coil to apply the magnetic field and produce inductioncurrents within the trunnion shoulder surface and trunnion surface ofthe article for a time sufficient to induce heating them to a heattreatment temperature (T_(H)) to at least a selected case depth; (5)withdrawing the trunnion from the induction coil at a selected rate; (6)cooling the trunnion surface and trunnion shoulder surface of thearticle to a temperature (T_(C)) to the selected case depth; and (7)repeating steps (2)-(6) for a selected number of the trunnions.

The present invention also includes an article, such as: a steel spidercomprising a hub, a plurality of angularly spaced trunnion shouldersextending from the hub, each having a trunnion shoulder surface, and acorresponding plurality of angularly-spaced trunnions extending from theplurality of trunnion shoulders, each trunnion having a trunnion axisand a trunnion surface, the trunnion surfaces and the trunnion shouldersurfaces comprising a hardened case, wherein the hardened case is formedby an induction heat treatment.

Certain difficulties associated with the inductive heat treatment ofcomponents having an irregular outer surface, such as the spider oftripod-type CV joints, have been overcome by the use of the method ofheat treatment and induction coil described herein.

The present invention undertakes to improve the production of suchcomponents as compared to previous methods, such as carburizing, byenabling the use induction hardening, and thereby providing bettercontrol over the hardening process by hardening one component at a time,improving the metallurgical and mechanical and properties of thecomponents, and allowing for a reduction in heat treatment cost.

The hardening operation will be simplified, and allow improved control,by the application of this invention because the components will beprocessed one at a time. The integration of the part location, heating,and quenching functions into a single, robust machine simplifies theheat treatment operation compared to previous methods, such ascarburizing, by reducing the part handling requirements and reducingcomplex cycle parameters (e.g. adjusting the entire process forpart-to-part variations in a batch of parts due to different temperatureand environmental conditions that exist in a large heat treatingfurnace) to a small set of control parameters for each individual part(e.g. power, induction time, quench flow rates, etc.). Enabling theautomatic control of process variables, such as the power level, totalpower delivered, quench temperature, quench flow rate, and cycle timingparameters, along with other process variables, will enable improvedprocess control.

The mechanical properties of the components may also be improved by theselective application of heat in only the areas where high hardness isdesired to give more precise control over the hardness and properties ofthe critical areas of the component while minimizing distortion from thehardening process.

Benefits from this invention include increased component strength (ascompared to components processed by conventional methods such ascarburizing), use of lower cost materials, shortened process times,reduced forging costs, reduced distortion, improved microstructures,improved tool life, deeper case depth capabilities, and the use ofcellular process lines.

Further scope of applicability of the present invention will becomeapparent from the following detailed description, claims, and drawings.However, it should be understood that the detailed description andspecific examples, while indicating preferred embodiments of theinvention, are given by way of illustration only, since various changesand modifications within the spirit and scope of the invention willbecome apparent to those skilled in the art.

BRIEF DESCRIPTION OF THE DRAWINGS

The present invention will become more fully understood from thedetailed description given below, the appended claims, and theaccompanying drawings in which:

FIG. 1 is a flow diagram illustrating the method of the invention.

FIG. 2 is a top view of an article of the present invention in the formof a spider.

FIG. 3 is a front view of the spider of FIG. 2.

FIG. 4 is a top view of an induction coil.

FIG. 5 is a cross-sectional view of the induction coil of FIG. 4 alongsection 5-5.

FIG. 6 is a bottom view of the induction coil of FIG. 4.

FIG. 7A is a cross-sectional view of an induction coil and spiderillustrating step 30 of the method of the invention.

FIG. 7B is a cross-sectional view of an induction coil and spiderillustrating step 40 of the method of the invention.

FIG. 7C is a cross-sectional view of an induction coil and spiderillustrating step 50 of the method of the invention.

FIG. 7D is a cross-sectional view of an induction coil and spiderillustrating step 60 of the method of the invention.

FIG. 7E is a cross-sectional view of an induction coil and spiderillustrating step 70 of the method of the invention.

FIG. 7F is also a cross-sectional view of an induction coil and spiderillustrating step 70 of the method of the invention.

FIG. 8 is a cross-sectional view of section 8-8 of FIG. 2.

FIG. 9 is a cross-sectional view of section 9-9 of FIG. 2.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

Referring to FIGS. 1-7, the present invention generally comprises amethod 1 of induction heat treatment, comprising the steps of: (1)selecting 10 an article 100, such as spider 200, for heat treatmentcomprising hub 210 having hub surface 215 and a plurality of angularlyspaced trunnions 220 extending from a corresponding plurality oftrunnion shoulders 225 formed in the hub surface 215, each trunnionshoulder 225 having a trunnion shoulder surface 245, and each trunnion220 having a trunnion axis 230 and a trunnion surface 235; (2) selecting20 an induction coil 300, which is adapted to receive a trunnion 220 forheat treatment and apply a magnetic field to the trunnion surface 235and trunnion shoulder surface 245; (3) placing 30 a trunnion 220 withinthe induction coil 300 with its corresponding trunnion shoulder 225adjacent to the induction coil 300; (4) rotating 40 the trunnion 220within the induction coil 300 about the trunnion axis 230 at a selectedspeed; (5) energizing 50 the induction coil 300 to apply the magneticfield and produce induction currents within the trunnion surface 235 andtrunnion shoulder surface 245 of the article 100 for a time sufficientto induce heating them to a heat treatment temperature (T_(H)) to atleast a selected case depth; (6) withdrawing 60 the trunnion from theinduction coil at a selected rate; (7) cooling 70 the trunnion surfaceand trunnion shoulder surface of the article to a temperature (T_(C)) tothe selected case 250 depth; and (8) repeating 80 steps (3)-(7) for aselected number of the trunnions.

With regard to the step of selecting 10 an article 100, this method ofinduction heat treatment is ideally suited for the induction hardeningheat treatment of articles 100, such as spider 200, comprising a hub 210having a plurality of angularly-spaced trunnions 220 or shafts extendingfrom the hub surface 215, as illustrated in FIGS. 2 and 3. Preferably,spider 200 is formed from an induction hardenable metal, such as amedium to high carbon steel having a microstructure comprising a mixtureof pearlite and ferrite. Induction hardenable steels are referred toherein as pearlitic/ferritic steels. Preferably, hub 210 and trunnions220 are formed from a single starting blank, such as by forging. Hub 210may be of any suitable shape, but is preferably generally cylindrical.Hub surface 215 may be any shape that is suitable to receive thetrunnions, but is preferably barrel-shaped or convex. It is alsopreferred that trunnion shoulders 225 be formed in hub surface 215 andextend outwardly from hub 210, and that trunnions 220 extend radiallyand outwardly from trunnion shoulders 225. Trunnions 220 may compriseany suitable shape, but preferably comprise a right circular cylinder asillustrated in FIGS. 2 and 3.

While it is believed that the method of the present invention may beused for the induction heat treatment 1 of a number of articles 100 ofthe type described above, FIGS. 2, 3, 8 and 9 illustrate a particularembodiment of article 100, comprising a spider 200 of a tripod typeconstant velocity joint that Applicants have induction heat-treatedusing method 1. Spider 200 was generally cylindrical, having a hub 210with a maximum diameter of about 100 mm and a thickness of about 40 mmand comprised AISI 1050 warm forged steel. Spider 200 comprised an outersurface 205, comprising hub surface 215, trunnion surfaces 235, andtrunnion shoulder surfaces 245, and a core 240. Hub surface 215 wasgenerally convex or barrel-shaped. Trunnion shoulders 225 were generallycylindrical, having a diameter of about 35 mm. In the embodimentillustrated in FIGS. 2 and 3, there were three trunnion shoulders 225,associated with three trunnions 220, however, the present invention isalso applicable to hubs 210 that contain more or less than threetrunnions 220 or shafts. Trunnions 220 were about 40 mm long and 30 mmin diameter. Hub also comprised a bore 250, which was a splined bore,and is the means for attaching spider 200 to an axle shaft (not shown).

Referring to FIGS. 8 and 9, it is an object of the induction heattreatment to form an induction-hardened case over the entirety oftrunnion surfaces 235 and trunnion shoulder surfaces 245. In the case ofspider 200, these surfaces are primarily subjected to loads associatedwith the transmission of torque from a drive shaft to a driven shaft asa CV joint incorporating spider 200 is translated and angulated duringoperation of a vehicle. Therefore, these surfaces have carefully definedmechanical requirements for strength, toughness and fatigue resistance,and corresponding microstructural requirements for hardness, case depth,phase constituents and distribution and other characteristics, such asthose described above.

Tripod joints are presently made by a number of manufacturers. Thisbeing the case, there are many variations in the particular features anddetails of tripod joints and their associated spiders 200, includingvariations of the size, including the thickness and diameter, the degreeand type of curvature of hub surface 215, the shape and size oftrunnions 220, the composition of the material and methods used to formspider 200, and other features. However, while some differences exist,most spiders comprise pearlitic/ferritic steels, and it is believed thatthe present invention is applicable to many of the spiders currentlybeing manufactured for tripod CV joints.

Having selected 10 article 100, such as spider 200, the method of heattreatment 1 comprised the additional step of selecting 20 an inductioncoil 300. Referring to FIGS. 4-6, the induction coil 300 selectedcomprised a cylindrical coil 300 having a cylindrical portion 302, atermination portion 304, and a longitudinal axis 306. Cylindricalportion 302 of induction coil 300 also preferably comprises an integralquench ring 308 that is fabricated so as to form an integral portion ofcylindrical portion 302. Due to the fact that quench ring 308 extendsinwardly of the inner sidewall of cylindrical portion 302, it also actsas a flux concentrator, such that the magnetic field is strongest andmost closely coupled to article 100 within the bore of quench ring 308.Referring again to FIGS. 4-6, induction coil 300 may comprise anysuitable size, cross-sectional shape and composition, depending on theexact nature of article 100 that is to be used therewith. However, inthe case of spider 200, induction coil 300 had an effective innerdiameter of about 38 mm and comprised a hollow, rectangular, copper tubehaving an internal width of 12.5 mm and an internal height of 12.5 mm,and a sidewall thickness of 2-3 mm. While many conductive materials maybe used for induction coil 300, it is preferably made from pure coppertubing, generally having a purity of at least 99%. Induction coil 300must be adapted so as to receive article 200, while preferablymaintaining as close a spacing as is practicable, so as to maximize theinductive coupling with article 100 when induction coil 300 isenergized, and yet not interfere with the rotation or withdrawal ofarticle 100, as discussed below. Induction coil 300 is preferablyadapted so that longitudinal axis 306 of coil 300 may be easily alignedto be parallel to and coincident with trunnion axes 230.

Induction coil 300 is also adapted to apply a planar magnetic field tothe trunnion surfaces 235 and trunnion shoulder surfaces 245 oftrunnions 220. By planar, it is meant that the centerline of themagnetic field that results when induction coil 300 is energized, whichroughly corresponds to the centerline of the tube, defines a plane.Referring to FIGS. 4-6, the magnetic field that is produced wheninduction coil 300 is energized may be described as being generallycylindrical, corresponding to the shape of cylindrical portion 302 ofinduction coil 300.

Referring to FIG. 7A, the next step of method 1 comprises placing 30 thetrunnions 220 of article 100, such as spider 200, within the inductioncoil 300. Placing 30 comprises providing a rotatable means for placing,rotating and withdrawing article 100 and performing the subsequent stepsof method 1. As discussed above and illustrated in FIG. 7A with regardto spider 200, spider 200 is preferably placed within induction coil 300so that trunnion axis 230 is parallel to and coincident withlongitudinal axis 306 of induction coil 300. Trunnion shoulder 225 isplaced adjacent to induction coil 300, such that the magnetic fieldproduced when induction coil 300 is energized is inductively coupled toboth the trunnion surface 235 and the trunnion shoulder surface 245.Spider 200 may be placed into induction coil 300 by any number ofsuitable known means for holding and rotating spider 200, such as arotatable and translatable jig or fixture. It is also preferable thatmeans for holding and rotating spider 200 be selected so as to minimizeany interference with the magnetic fields generated by induction coil300.

Referring to FIG. 7B, the next step of method 1 comprises rotating 40spider 200 within induction coil 300 at a selected speed. This speed maybe any suitable speed and may comprise a variable speed during or withinthe subsequent steps of method 1. Rotation is used to compensate for thefact that induction coil 300 has a region where the return legs 312 and314 of termination portion 304 and generally cylindrical portion 302meet where the resultant magnetic field is non-uniform and generallyreduced as compared to adjacent sections of induction coil 300. In thecase of the application of method 1 to spider 200 described herein, therotational speed was about 100-200 rpm, preferably about 150 rpm.

Referring to FIG. 7C, the next step of method 1 comprises energizing 50the induction coil 300 to a selected energy level to apply the magneticfield and produce an induction current within trunnion surface 235 andtrunnion shoulder surface 245. In the case of steel, such as AISI 1050steel, to provide induction hardening, this energizing 50 must beperformed for a time sufficient to induce heating of these surfaces to aheat treatment temperature (T_(H)) to at least a selected case 250depth, such as the required or desired hardened case 250 depth. Asillustrated in FIG. 7C, in the case of spider 200, and induction coil300, the step of energizing 50 comprised applying 60% power from acommercially available 60 kW power supply of a type used for inductionheat treatment in a range of about 350-400 kHz and preferably about 400kHz. In the case of spider and trunnions, this step of energizing 50 wassufficient to heat all of trunnion surface 235 and trunnion shouldersurface 245 to a temperature that was above the austenite transitiontemperature to selected case 250 depth of at least 1 mm. The austenitetransition temperature for the AISI 1050 material is about 1700-2000° F.The actual depth of the heat treatment ranged from about 1-2 mm. It willbe readily understood that the inductive frequency and power can bealtered depending on the size, shape, composition and other factorsassociated with trunnion 220, the specific design of inductor coil 300,as well as other factors.

Referring to FIGS. 7D-F, the next step of method 1 comprises withdrawing60 the trunnion 220 from the induction coil 300 at a selected rate alongits longitudinal axis 306 so as to scan trunnion 220 within inductioncoil 300 and gradually withdraw trunnion 220 from induction coil 300. Inthe case of spider 200, the withdrawal or scan rate of the trunnion wasabout 0.098 cm/sec. Further, it is preferred that the steps ofenergizing 50, withdrawing 60 and cooling 70 be coordinated to provide adwell at the initial placement position, in order that that trunnionshoulder surface 245 be heated above the austenite transitiontemperature prior to initiating the step of withdrawing 60 and cooling70. For example, in the case of spider 200, the dwell was about threeseconds, after which trunnion 220 was withdrawn 60 to the position shownin FIG. 7E, whereupon cooling 70 was initiated while trunnion 220 waswithdrawn 60 to the position shown in FIG. 7F.

The next step of method 1 comprises cooling 70 trunnion surface 235 andtrunnion shoulder surface 245 of article 100 to a temperature (T_(C)) tothe selected case 250 depth. This temperature (T_(C)) can be anytemperature that is lower than the heat treatment temperature (T_(H)),but typically will be selected to produce certain desired transformationproducts within case 250. In the case of spider 200, the desiredtransformation product in case was martensite, hence, T_(C) was selectedto be below the martensite transformation temperature, which in the caseof AISI 1050 was about 200° F. Cooling 70 comprised quenching trunnionin an aqueous quenchant comprising 3-5% of a commercially availablepolymer quenchant additive, Aqua Quench 251, for a time sufficient tocool trunnion surface 235 and trunnion shoulder surface 245 below T_(C).Quenching was accomplished by pumping a large volume of the quenchantthrough inductor coil 300 and quench ring 308 onto the trunnion surface235 and trunnion shoulder surface 245. Quenching 70 was accomplishedusing quench ring 308 having a plurality of spray holes 310 in the lowersurface of quench ring that were directed radially inwardly anddownwardly towards longitudinal axis 306 of induction coil 200 as shownin FIGS. 7D and 7E. The quench time corresponded with the scan oftrunnion 220 within induction coil 300, and for spider 200 was about 5seconds. The quenchant flow rate was about 10 gpm.

Referring to FIGS. 8 and 9, following the step of cooling 70, thesurface hardness of trunnion surface 235 and trunnion shoulder surface245 were in the range of R_(C)58-63, with a hardened case depth range ofapproximately 1-2 mm effective at R_(C)50, and a core 240 hardness ofR_(C)15-30. The microstructure comprised martensite in trunnion surface235 and trunnion shoulder surface 245 and case 250, and fine grains ofpearlite and ferrite in core 240.

Applicants believe that it is also possible to use method 1 to alsoproduce a tempered martensite structure in case 250 by controlling thestep of cooling 70 so that trunnion surface 235 and trunnion shouldersurface 245 are cooled by quenching such that T_(C) is in a range thatis below the martensite start temperature (about 610° F.) but greaterthan the martensite finish temperature (about 200° F.), and thenpermitting the part to cool under ambient conditions, such that themartensitic structure is tempered by the residual heat in article 100 bymeans of the reduced cooling rate. Quench composition and concentration,temperature, flow rates and time are adjusted to allow the use ofresidual heat to sufficiently temper (stress relieve) the part, therebyeliminating the need for secondary tempering processing. It is believedthat this will reduce the residual stresses in case 250, as well as thehardness to a range of about R_(C)58-63. Applicants believe that thiscan be accomplished by shielding previously heat-treated trunnions fromsubsequent quenching 70, such as by use of a quench shield 320 as shownin phantom in FIG. 5.

Following the step of cooling 70, method 1 comprises repeating steps 30through 70 for a selected number of trunnions 220. In the case of spider200, it is preferable to perform these steps on all three trunnions 220.However, is it possible to apply method 1 to some or all of trunnions220. Further, it is will be appreciated that the heat treatment for eachtrunnion 220 need not be the same, if the design criteria for trunnions220 are not the same. Further, the heat treatment can be varied alongthe length of each of trunnions 220 and trunnion surfaces 235 should thedesign criteria so require.

Following induction heat treatment 1, spider 200 may optionally be hardturned to produce the finished dimensions of article 100.

The foregoing discussion discloses and describes an exemplary embodimentof the present invention. One skilled in the art will readily recognizefrom such discussion, and from the accompanying drawings and claims thatvarious changes, modifications and variations can be made thereinwithout departing from the true spirit and fair scope of the inventionas defined by the following claims.

1. A method of induction heat treatment, comprising the steps of: (1)selecting an article for heat treatment comprising a hub having a hubsurface and a plurality of angularly spaced trunnions extending from acorresponding plurality of trunnion shoulders formed in the hub surface,each trunnion shoulder having a trunnion shoulder surface, and eachtrunnion having a trunnion axis and a trunnion surface; (2) selecting aninduction coil, which is adapted to receive a trunnion for heattreatment and apply a magnetic field to the trunnion surface andtrunnion shoulder surface; (3) placing a trunnion within the inductioncoil with its corresponding trunnion shoulder adjacent to the inductioncoil; (4) rotating the trunnion within the induction coil about thetrunnion axis at a selected speed; (5) energizing the induction coil toapply the magnetic field and produce induction currents within thetrunnion shoulder surface and trunnion surface of the article for a timesufficient to induce heating them to a heat treatment temperature(T_(H)) to at least a selected case depth; (6) withdrawing the trunnionfrom the induction coil at a selected rate; (7) cooling the trunnionsurface and the trunnion shoulder surface of the article to atemperature (T_(C)) to the selected case depth; and (8) repeating steps(3)-(7) for a selected number of the trunnions.
 2. The method of claim1, wherein the article comprises a spider.
 3. The method of claim 2,wherein the spider has a barrel-shaped outer surface, a plurality ofangularly spaced cylindrical shoulders extending from the outer surface,and a plurality of cylindrical trunnions extending from the cylindricalshoulders.
 4. The method of claim 2, wherein the spider comprises apearlitic/ferritic steel.
 5. The method of claim 4, wherein the steelcomprises AISI 1050 steel.
 6. The method of claim 4, wherein T_(H) isgreater than the austenite transition temperature.
 7. The method ofclaim 6, wherein the T_(H) is in the range of 1700-2000° F.
 8. Themethod of claim 6, wherein said step of cooling comprises quenching thetrunnion.
 9. The method of claim 8, wherein T_(C) is less than themartensite start temperature and greater than the martensite finishtemperature.
 10. The method of claim 9, further comprising stopping thequenching when the trunnion surface and the trunnion shoulder surface ofthe spider is less than or equal to T_(C) to the selected case depth,and then permitting the trunnion surface to cool under ambientconditions.
 11. The method of claim 10, further comprising the step of(9) shielding any previously heated trunnion surface and trunnionshoulder surface during step (7) from additional quenching, wherein allof the trunnion surfaces are cooled from T_(C) under ambient conditions.12. A method of induction heat treatment of a spider having abarrel-shaped outer surface, a plurality of cylindrical trunnionshoulders formed in the outer surface of the hub and a correspondingplurality of angularly spaced cylindrical trunnions extending from theshoulders, each trunnion shoulder having a trunnion shoulder surface,and each trunnion having a trunnion axis and a trunnion surface,comprising the steps of: (1) selecting an induction coil, which isadapted to receive a trunnion for heat treatment and apply a magneticfield to the trunnion surface and the trunnion shoulder surface; (2)placing a trunnion within the induction coil with its correspondingtrunnion shoulder adjacent to the induction coil; (3) rotating thetrunnion within the induction coil about the trunnion axis at a selectedspeed; (4) energizing the induction coil to apply the magnetic field andproduce induction currents within the trunnion shoulder surface andtrunnion surface of the article for a time sufficient to induce heatingthem to a heat treatment temperature (T_(H)) to at least a selected casedepth; (5) withdrawing the trunnion from the induction coil at aselected rate; (6) cooling the trunnion surface and the trunnionshoulder surface of the article to a temperature (T_(C)) to the selectedcase depth; and (7) repeating steps (2)-(6) for a selected number of thetrunnions.
 13. The method of claim 12, wherein the spider comprises apearlitic/ferritic steel.
 14. The method of claim 13, wherein the steelcomprises AISI 1050 steel.
 15. The method of claim 13, wherein T_(H) isgreater than the austenite transition temperature.
 16. The method ofclaim 15, wherein T_(H) is in the range of 1700-2000° F.
 17. The methodof claim 15, wherein said step of cooling comprises quenching thearticle.
 18. The method of claim 17, wherein said step of coolingcomprises quenching until T_(C) is lower than the martensite starttemperature.
 19. The method of claim 18, wherein T_(C) is less than themartensite start temperature and greater than the martensite finishtemperature.
 20. The method of claim 19, further comprising stopping thequenching when the temperature of the trunnion surface and the trunnionshoulder surface reaches T_(C) to the selected case depth, and thenpermitting the spider to cool under ambient conditions.
 21. The methodof claim 20, further comprising the step of (8) shielding any previouslyheated trunnion surface during step (6) from additional quenching,wherein all of the trunnion surfaces and trunnion shoulder surfaces arecooled under ambient conditions.
 22. An article, comprising: a steelspider comprising a hub, a plurality of angularly spaced trunnionshoulders extending from the hub, each having a trunnion shouldersurface, and a corresponding plurality of angularly spaced trunnionsextending from the plurality of trunnion shoulders, each trunnion havinga trunnion axis and a trunnion surface, the trunnion surfaces and thetrunnion shoulder surfaces comprising a hardened case, wherein thehardened case is formed by an induction heat treatment.
 23. The articleof claim 22, wherein the induction heat treatment comprises the steps of(1) selecting an induction coil, which is adapted to receive a trunnionfor heat treatment and apply a magnetic field to the trunnion surfaceand the trunnion shoulder surface; (2) placing a trunnion within theinduction coil with its corresponding trunnion shoulder adjacent to theinduction coil; (3) rotating the trunnion within the induction coilabout the trunnion axis at a selected speed; (4) energizing theinduction coil to apply the magnetic field and produce inductioncurrents within the trunnion surface and trunnion shoulder surface ofthe article for a time sufficient to induce heating them to a heattreatment temperature (T_(H)) to at least a selected case depth; (5)withdrawing the trunnion from the induction coil at a selected rate; (6)cooling the trunnion surface and the trunnion shoulder surface of thearticle to a temperature (T_(C)) to the selected case depth; and (7)repeating steps (2)-(6) for a selected number of the trunnions.
 24. Thearticle of claim 23, wherein the induction hardened case comprises amartensitic microstructure and the core comprises a microstructure thatis a mixture of pearlite and ferrite.
 25. The article of claim 24,wherein the induction hardened case has a hardness of about R_(C)58-63,and the core has a hardness of about R_(C)15-30.
 26. The article ofclaim 24, wherein the martensitic microstructure is a temperedmartensitic microstructure.
 27. The article of claim 26, wherein thetempered martensitic microstructure is formed by the induction heattreatment.
 28. The article of claim 27, wherein the tempered martensiticmicrostructure has a hardness of about R_(C)58-63.
 29. The article ofclaim 28, wherein the depth of the case is about 1-2 mm.